Electric motor having axially centered ball bearings

ABSTRACT

A bearing design for a rotatable assembly includes two freely rotating balls mounted on the axis of rotation of the assembly and axially separated, one near each axial end of the assembly. Each ball is confined by a moving concave (preferably conical or frustro-conical) bearing surface of the rotatable assembly and a corresponding fixed concave bearing surface of a mounting attached to a frame, housing, or similar non-rotating structure. One of the fixed mountings is preferably attached to a compressible spring to provide a controlled axial pre-load to the assembly. The balls are substantially enclosed and lubricant provided in the enclosed cavity. In the preferred embodiment, the rotatable assembly is a rotary actuator assembly of a disk drive. Compared to conventional ball bearing designs, the present design reduces the number of parts and volume of space occupied by the bearings, reduces hysteresis, and improves shock resistance.

This is a divisional of application Ser. No. 08/446,381 filed on May 22,1995 now U.S. Pat. No. 5,835,309.

FIELD OF THE INVENTION

The present invention relates to bearings used to support rotatableassemblies, and in particular to bearings used in disk drive actuatorsand spindles.

BACKGROUND OF THE INVENTION

The extensive data storage needs of modern computer systems requirelarge capacity mass data storage devices. A common storage device is therotating magnetic disk drive.

A disk drive typically contains one or more smooth, flat disks which arerigidly attached to a common spindle. The disks are stacked on thespindle parallel to each other and spaced apart so that they do nottouch. The disks and spindle are rotated in unison at a constant speedby a spindle motor.

Each disk is formed of a solid disk-shaped base or substrate, having ahole in the middle for the spindle. The substrate is commonly aluminum,although glass, ceramic, plastic or other materials arc possible. Thesubstrate is coated with a thin layer of magnetizable material, and mayadditionally be coated with a protective layer.

Data is recorded on the surfaces of the disks in the magnetizable layer.To do this, minute magnetized patterns representing the data are formedin the magnetizable layer. The data patterns are usually arranged incircular concentric tracks. Each track is further divided into a numberof sectors. Each sector thus forms an arc, all the sectors of a trackcompleting a circle.

A movable actuator positions a transducer head adjacent the data on thesurface to read or write data. Although earlier disk drive designs useda linear actuator, which moved back and forth on straight rails, mostdisk drives now being produced use a rotary actuator, which pivots aboutan axis. The rotary actuator may be likened to the tone arm of aphonograph player, and the head to the playing needle.

There is one transducer head for each disk surface containing data. Thetransducer head is an aerodynamically shaped block of material (usuallyceramic) on which is mounted a magnetic read/write transducer. Theblock, or slider, flies above the surface of the disk at an extremelysmall distance as the disk rotates. The close proximity to the disksurface is critical in enabling the transducer to read from or write tothe data patterns in the magnetizable layer. Several differenttransducer designs are used, and in some cases the read transducer isseparate from the write transducer.

A rotary actuator typically includes a solid block near the axis havingcomb-like arms extending toward the disk, a set of thin suspensionsattached to the arms, and an electro-magnetic motor on the opposite sideof the axis. The transducer heads are attached to the suspensions, onehead for each suspension. The actuator motor rotates the actuator toposition the head over a desired data track. Once the head is positionedover the track, the constant rotation of the disk will eventually bringthe desired sector adjacent the head, and the data can then be read orwritten.

As computer systems have become more powerful, faster, and morereliable, there has been a corresponding increase in demand for improvedstorage devices. These desired improvements take several forms. It isdesirable to reduce cost, to increase data capacity, to increase thespeed at which the drives operate, to reduce the electrical powerconsumed by the drives, and to increase the resilience of the drives inthe presence of mechanical shock and other disturbances.

In particular, there is a demand to reduce the physical size of diskdrives. To some degree, reduction in size may serve to further some ofthe above goals. But at the same time, reduced size of disk drives isdesirable in and of itself. Reduced size makes it practical to includemagnetic disk drives in a range of portable applications, such as laptopcomputers, mobile pagers, and “smart cards”.

An example of size reduction is the application of the PCMCIA Type IIstandard to disk drives. This standard was originally intended forsemiconductor plug-in devices. With improvements to miniaturizationtechnology, it will be possible to construct disk drives conforming tothe PCMCIA Type II standard.

In order to shrink the size of disk drives, every component must bereduced in size as much as possible. Additionally, because the PCMCIAType II standard, as well as many other small form factor drives, areintended for portable use, it is necessary that such devices be capableof tolerating a high mechanical shock, such as might occur when a diskdrive is dropped onto a hard floor. Conventional drives designed fordesktop applications have been susceptible to shock damage. Withportable applications becoming more significant, there is a need to findnew design techniques to permit reduced size and power consumption, tomake assembly of miniaturized components practical, and prevent thedrive from being damaged when exposed to mechanical shock.

Conventionally, the rotatable disk spindle assembly and the rotaryactuator assembly are supported by sets of ball bearings housed inannular races. Typically, there are two sets of bearings for the diskspindle and two for the rotary actuator, the two sets supporting aparticular assembly being axially separated to provide greaterstability. The number of parts makes it increasingly difficult to shrinkthe size of the bearing assembly. Additionally, when this design isminiaturized for a small form factor disk, individual balls becomeextremely small and susceptible to mechanical shock. Finally, multipleballs generate significant bearing drag and mechanical hysteresis, thelatter being particularly troublesome for rotary actuators, whichfrequently alter direction.

It has been proposed to address problems of miniaturization of spindlebearings by using fluid or hydrodynamic bearings. Such bearing designscould potentially reduce parts, permit greater speeds, and enhance shockresistance of spindle bearings. However, oil containment in such alimited space is a major problem which has yet to be completelyovercome. Additionally, proper operation of a fluid bearing requirescontinuous, high speed rotation. Disk spindles, which typically rotateat high constant speed, may become suitable applications. But a rotarydisk actuator typically moves back and forth in a short arc. The motionof a disk actuator would generally not produce sufficient fluid pressureto support a fluid bearing, and fluid bearings would therefore beunsuitable for disk actuator assemblies.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide anenhanced bearing for, supporting pivotable assemblies.

Another object of the present invention to provide an enhanced diskdrive storage apparatus.

Another object of this invention is to reduce the cost of a disk drivestorage apparatus.

Another object of this invention is to reduce the number of parts in abearing assembly, particularly a bearing assembly of a disk drivestorage apparatus.

Another object of this invention is to provide a disk drive storageapparatus which is easier to fabricate and assemble.

Another object of this invention is to reduce the volume of a bearingand pivotable assembly.

Another object of this invention is to provide a bearing assembly havinggreater resistance to mechanical shock.

Another object of this invention is to reduce the size of a disk drivestorage apparatus.

Another object of this invention is to provide a disk drive storagedevice having greater resistance to mechanical shock.

Another object of this invention is to reduce the amount of powerrequired to operate a rotatable assembly, and in particular a rotatableassembly of a disk drive storage device.

A bearing design for a rotatable assembly includes two freely rotatingballs mounted on the axis of rotation of the assembly and axiallyseparated, one near each axial end of the assembly. Each ball isconfined by a moving concave bearing surface of the rotatable assemblyand a corresponding fixed concave bearing surface of a mounting attachedto a frame, housing, or similar non-rotating structure.

In the preferred embodiment, the rotatable assembly is a rotary actuatorassembly of a disk drive. The bearing surfaces are preferably concavesurfaces defining an inner space in the shape of a cone or frustum of acone, which is centered about the axis of rotation. The ball, which ispreferably spherical, occupies part of the spaces defined by the concavebearing surfaces. One of the fixed mountings is attached to acompressible spring to provide a controlled axial pre-load to theassembly. The actuator assembly and actuator mounting on the housingmate to substantially enclose the ball. A lubricant is provided in theenclosed space. The space is sealed with an O-ring.

Compared to conventional ball bearing designs for actuators andspindles, the present invention provides numerous advantages. It reducesthe number of parts and the volume of space occupied by the bearings.The reduction in number of balls will reduce hysteresis and bearingdrag, reducing power requirements of the actuator motor. At the sametime, the ball itself, and the contact area between the ball and bearingsurfaces, is much larger, making the bearing more shock resistant.

Various alternative embodiments of the present invention are possible.For example, the bearing may be used to support the spindle motor of adisk drive. It may be used in any of numerous applications where reducedsize of the assembly is an important consideration, includingmicromotors (size in microns).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a magnetic disk drive storage unit according to thepreferred embodiment;

FIG. 2 is a partial cross-sectional view of the magnetic disk drive,showing the actuator assembly, according to the preferred embodiment;

FIG. 3 is a top view of the pre-load spring of the preferred embodiment;

FIG. 4 is an enlarged cross-sectional view of a single ball andcorresponding conical bearing surfaces according to the preferredembodiment;

FIG. 5 is a cross-sectional view of a first alternative embodiment of aball and corresponding bearing surfaces, showing arched bearingsurfaces;

FIG. 6 is a cross-sectional view of a second alternative embodiment of aball and corresponding bearing surfaces, showing spherical bearingsurfaces;

FIG. 7 is a cross-sectional view of a third alternative embodiment of aball and corresponding bearing surfaces, showing a combination ofconical and spherical bearing surfaces;

FIG. 8 is a cross-sectional view of a motor assembly employing thebearing assembly according to an alternative embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a partially exploded view of a magnetic disk drive storageunit 100 in accordance with the preferred embodiment. Disk unit 100comprises rotatable disk 101, which is rigidly attached to hub 103. Hub103 is rotatably mounted on disk drive base 104. Hub 103 and disk 101are driven by a drive motor at a constant rotational velocity. The drivemotor is contained within hub 103. Actuator assembly 105 is situated toone side of disk 101. Actuator 105 pivots through an arc about axis 106parallel to the axis of the spindle to position the transducer heads.Actuator 105 is driven by an electro-magnetic motor comprising a set ofpermanent stationary magnets 110 rigidly attached to base 104, and anelectromagnetic coil 111 attached to the actuator. Cover 115 mates withbase 104 to form a complete enclosure or housing, to protect the diskand actuator assemblies. Electronic modules for controlling theoperation of the drive and communicating with another device, such as ahost computer, are mounted on a circuit card 112 within the head/diskenclosure formed by base 104 and cover 115. In this embodiment, circuitcard 112 is mounted within the enclosure and shaped to take up unusedspace around the disk in order to conserve space, as would be used for aPCMCIA Type II form factor. However, the card 112 could also be mountedoutside the head/disk enclosure, or the base itself could be made as acircuit card for mounting electronic modules directly to it. A pluralityof head/suspension assemblies 108 are rigidly attached to the prongs ofactuator 105. An aerodynamic read/write transducer head 109 is locatedat the end of each head/suspension assembly 108 adjacent the disksurface.

Actuator 105 pivots on a pair of spherical balls, one of which is shownin FIG. 1 as feature 201. The balls are confined by respective mounting(one shown in FIG. 1 as feature 220) and concave surfaces of theactuator. Mounting 220 is attached to a compressible spring 224, whichis positioned within a relief in the inner surface of cover 115.

While only a single disk is shown in the preferred embodiment (as wouldbe typical of a PCMCIA type II form factor disk drive), is should beunderstood that the number of disks mounted on hub 103 may vary.

FIG. 2 is a partial cross-sectional view of disk drive 100, taken in theplane of actuator axis 106, showing more clearly certain components ofthe actuator bearing assembly. For ease of orientation, actuator 105 isshown with attached suspensions 108 and transducer heads 109 whichaccess data on disk 101, on one side of axis 106. On the other side ofaxis 106 is the actuator motor, comprising stationary magnets 110attached to base 104 and coil 111 attached to actuator 105.

The actuator bearing assembly comprises two freely rotating sphericalballs 201, 202, both of which are centered on actuator axis 106 andaxially separated. Each ball 201, 202 is confined by a respective pairof concave bearing surfaces. Bearing surfaces 210, 211 confine ball 201,and bearing surfaces 212, 213 confine ball 202. Preferably, each bearingsurface 210-213 is conical or frustro-conical in shape, the cones beingcentered on axis 106.

Bearing surfaces 210, 212 are preferably machined inner surfaces ofrespective cylindrical mounting blocks 220, 222. Mountings 220, 222 arestationary with respect to the disk base 104, i.e., they do not pivotwith the actuator. In the preferred embodiment, lower mounting block 222is rigidly attached to base 104 by press fit in a corresponding reliefcavity, or by attachment with adhesive, screws, or other appropriatemeans. Alternatively, block 222 could be an integral part of a basecasting, in which surface 212 is machined or otherwise formed. Uppermounting 220 is preferably attached to compressible pre-load spring 224with a suitable adhesive. Spring 224 is in turn attached to cover 115with adhesive or by press fit into a relief cavity in the cover.

Mountings 220,222 preferably include respective hollow cylindricalshroud portions 226,227 which extend over and encircle the matingportions of actuator 105. A pair of O-rings 228,229 are positionedsurrounding the axis 106, within circumferential grooves of the actuatorand shroud portions. O-rings 228,229 seal the cavities in which balls201,202 are located. To reduce friction, O-rings 228,229 wouldpreferably not be in contact with shroud portions 226,227, but theirclose proximity to the shrouds forms a labyrinth seal of the cavities. Alubricant (not shown) is placed in the cavities before sealing.

Bearing surfaces 211,213 are preferably machined inner surfaces of shaftmember 225. Shaft member 225 comprises upper mounting portion 221 havingbearing surface 211 for contacting ball 201, and lower mounting portion223 having bearing surface 213 for contacting ball 202. In the preferredembodiment, shaft member 225 is a separate member which is rigidlyattached to actuator 105, allowing shaft 225 to be formed of differentmaterials than used in actuator 105. Actuator 105 is preferablyaluminum, while shaft 225 is preferably steel. However, shaft 225 andactuator 105 could be integrally formed, or mounting portions 221,223could be separate members which are individually attached to actuator105 or shaft 225.

In a disk drive, it is highly desirable to obtain precise orientation ofthe actuator in order to accommodate the high track densities of typicaldrives. For this purpose, the bearings should be pre-loaded to restrictwobble of the actuator. In the preferred embodiment, pre-loading of thebearings is accomplished by imparting an axial force to mounting 220with compressible pre-load spring 224. Because actuator 105 floatsfreely between balls 201,202, the axial force is transmitted throughball 201, actuator 105, and ball 202, into mounting 222. The pre-loadforces ball 201 against conical bearing surfaces 210,211 and forces ball202 against conical bearing surfaces 212,213, firmly centering actuator105 on axis 106.

FIG. 3 is a top view of pre-load spring 224. Spring 224 is preferably aradially symmetrical member formed by stamping from stainless steelsheet having an appropriate thickness. Spring 224 includes a solidcentral portion 301 for attachment to mounting block 220 and a pluralityof curved arms 302, radially extending from the central portion to anouter perimeter 303. Outer perimeter 303 is attached to cover 115. Thearms are permanently deformed in the stamping process so that centralportion 301 and outer perimeter 303 lie in parallel planes offset fromeach other. This design provides a compressible spring having a verysmall vertical dimension.

Balls 201,202 contact bearing surfaces 210-213 along annular portions ofthe surface and balls. Conical surfaces 210-213 form an angle withrespect to a plane perpendicular to actuator axis 106. This angle isshown in FIG. 4 for bearing surface 210, and designated alpha (α). Theselection of an appropriate angle α will involve various engineeringdecisions. As the angle becomes shallower, the ball annular area ofcontact is reduced in radius; as the angle becomes steeper, the annulararea of contact is increased in radius. The consequence of this is thata steeper bearing surface angle increases stability, but also increasesfriction. An angle of approximately 45 degrees is preferred as areasonable trade-off between high stability and low friction. In thepreferred embodiment, all bearing surfaces form the same angle. However,it would be possible to construct the bearing assembly of the presentinvention with differing angles, or where the bearing surfaces arearched or spherical, with differing radii. The rotational velocity ofthe balls will generally be somewhere between zero and the rotationalvelocity of the actuator or other rotating member. This rotationalvelocity may also be adjusted by varying the relative contact angles ofthe bearing surfaces.

When compared with a conventional actuator design in which a pluralityof balls are placed in an annular race surrounding a shaft, the presentdesign significantly improves shock resistance. In the conventionaldesign, the only area of contact between the balls and the race are asmall point on each individual ball. In the event of a significantshock, all the shock load is transmitted through these small points ofcontact. This can cause very high stresses at these points, and mayresult in permanent deformation of the race and/or ball. The presentdesign improves shock resistance by substantially increasing the area ofcontact. Rather than being a plurality of small discrete areas on eachball, the area of contact in the present design is a continuous annulararea on the bearing surface surrounding the axis. A slight elasticdeformation of the surface in the event of shock causes a very largeincrease in the contact area to reduce stress, and thus avoids permanentdeformation of the surface.

Because the bearing design of the present invention increases thecontact area when compared with a conventional design using multipleballs in an annular race, there may be an increased tendency formaterials to bond at the contact surfaces when lying idle for periods oftime. Accordingly it is preferred that balls 201,202 be made ofdifferent materials than those used in mountings. Specifically, balls201,202 are preferably formed of either ceramic or stainless steelJ2100. Ceramic is the preferred material where it is not necessary toform an electrical ground path from the actuator to the base; where aground path is needed, stainless steel is preferred. Mountings arepreferably formed of hardened common steel 440C.

Many possible alternative combinations of materials for the balls andmountings are possible. For example, bronze is frequently an appropriatematerial for a mounting. It would even be possible to use polymericmaterials in some applications, although polymers would probably beinappropriate for most disk drives. It would be possible to make ballsand mountings of the same materials, although generally it is preferredthat the balls be of a different and harder material than the mountings.

Both the stationery mountings and the mountings on the actuator could beseparate parts as shown in FIG. 2, or could be integral with the base,cover or actuator. The actuator is typically formed of aluminum ormagnesium, and it would be possible to machine the bearing surfaces incorresponding integral cylindrical projections of the actuator body,centered on the axis of rotation of the actuator. The base is typicallyaluminum, and bearing surfaces could similarly be machined incorresponding projections from the base or cover. As used herein,“mounting” refers to that portion of the assembly which contains thebearing surface, and includes both integral mountings and mountingswhich are separate pieces attached by adhesive, press fit, or othermeans.

As will be observed from the above description, the bearing of thepreferred embodiment confines two freely moving axially centered ballswithin corresponding concave bearing surfaces. In the preferredembodiment, the concave surfaces are conical or frustro-conical,providing respective annular contact surfaces with the balls. However,many variations of the bearing surfaces are possible within the spiritand scope of the present invention.

FIG. 4 shows an enlarged cross-sectional view of a single ball 201 andcorresponding conical bearing surfaces 210-211 as used in the preferredembodiment, in the plane of the actuator axis. In the sectional view,one can observe four points of contact 401A, 401B, 402A, 402B, betweenthe ball and the bearing surfaces. In reality, these are not discretepoints. “Points” 401A, 401B are really opposite ends of an annularcontact area seen in cross section (and similarly 402A, 402B). The twoannular contact areas surround and are centered on the axis. Assuming aperfectly spherical ball and perfectly conical bearing surfaces, theannular contact areas have zero radial width. However, there will alwaysbe some width to the annular contact area because the ball and bearingsurfaces deform very slightly as a result of pre-load force, weight ofassembly, dynamic loading, etc.

FIG. 5 shows an alternative embodiment of a ball 501 and correspondingbearing surfaces 510-511. In the alternative embodiment of FIG. 5, thebearing surfaces are curved at the point of contact with the ball. Thiscurvature tends to increase the contact area, particularly when thebearing assembly is subjected to a severe load which elastically deformsthe ball and mountings. I.e., the radial width of the annulus whichforms the contact area would increase under load more rapidly than inthe embodiment of FIG. 4. The alternative embodiment of FIG. 5 wouldtherefore have a greater resistance to mechanical shock than theembodiment of FIG. 4. However, the embodiment of FIG. 5 would havecertain drawbacks vis-a-vis that of FIG. 4, particularly, the increasedcontact area would likely increase the force required to overcome staticfriction (stiction) upon initial movement, and may increase drag duringoperation as well.

FIG. 6 shows another alternative embodiment of a ball 601 andcorresponding bearing surfaces 610-611. In the embodiment of FIG. 6, thebearing surfaces 610-611 form part of the inner surface of a sphere, theradius of the sphere defining the bearing surfaces being larger than theradius of the ball. As a result, ball 601 contacts bearing surfaces610-611 at respective circular areas centered on the axis, rather thanat annular areas. Under conditions of low axial compressive force, thesecircular areas become very small (almost points), resulting in very lowstiction and drag. The contact areas increase in size when the assemblyis subjected to shock loading. However, the embodiment of FIG. 6 is notlikely to have equivalent shock resistance to those of FIGS. 4 or 5,since the contact area is much closer to the axis. Furthermore, theembodiment of FIG. 6 will have a lower accuracy of alignment of therotatable assembly than those of FIGS. 4 or 5, and may have more of atendency to wobble.

FIG. 7 shows yet another alternative embodiment of a ball 701 andcorresponding bearing surfaces 710-711. The embodiment of FIG. 7 is ahybrid of the embodiments of FIGS. 4 and 6. One of the bearing surfaces711 is conical as in FIG. 4, forming an annular area of contact withball 701. The other bearing surface 710 is spherical as in FIG. 6,forming a circular area of contact at the axis. The embodiment of FIG. 7is a compromise between the characteristics of the two constituentembodiments. It will have a lower stiction and drag than the embodimentof FIG. 4, although not as low as that of FIG. 6. It will also have moreaccurate alignment and higher shock resistance than the embodiment ofFIG. 6, although not as good as that of FIG. 4.

Just as certain operating parameters of the preferred embodiment may bevaried by varying the angles of the bearing surfaces, certain parametersof the embodiments of FIGS. 6 and 7 can be varied by varying the radiiof the bearing surfaces. Specifically, a larger radius will tend toreduce the contact area, reducing drag and alignment accuracy. A smallerradius will tend to increase the contact area, improving alignmentaccuracy but increasing drag.

In the preferred embodiment, the pivot bearing is used to support anactuator for a disk drive. The bearing is particularly well suited toactuators, because actuators pivot back and forth. The mechanicalresistance of an actuator having a conventional set of bearings andlubricant exhibits some hysteresis, due to the interaction of multipleballs and lubricant. This becomes more significant as the size of thedisk drive (and actuator) is reduced. The design disclosed hereinreduces this hysteresis effect because each bearing has only a singleball. However, such a bearing could also be used to support the diskspindle and rotor of a disk drive, particularly a small form factor diskdrive, even though the spindle rotates in only one direction. Theadvantages of reduced size, part count, and susceptibility to mechanicalshock are equally applicable to actuator bearings and disk spindlebearings.

FIG. 8 is a cross-sectional view of a motor assembly employing thebearing assembly according to such an alternative embodiment. While themotor shown in FIG. 8 is being used as the spindle drive motor of a diskstorage device, it should be understood that similar motors employingthe bearing assembly of the present invention could be used in otherapplications. The sectional view of FIG. 8 is taken in the plane of theaxis of rotation 803 of the disk. A pair of spherical balls 801,802 arepositioned centered on axis 803 and axially separated by rotatinghousing 806. Lower stationary mounting 822 is rigidly attached to base804 by adhesive or press fit. Upper stationary mounting 820 is attachedto pre-load spring 824 with a suitable adhesive, which is in turnattached to cover 805.

Housing 806 includes integral mounting portions 821,823 at opposite endsof axis 803, having respective bearing surfaces 811,813 in contact withballs 801,802. While mounting portions 821, 823 are integral with thehousing in the embodiment of FIG. 8, it should be understood that themountings could be separate members attached to the housing.Corresponding bearing surfaces 810,812 of stationary mountings 820,822contact balls 801,802 from opposite directions. Pre-load spring 824imparts a controlled axial pre-load force through upper mounting 820,ball 801, housing 806, and ball 802. The pre-load force in conjunctionwith the concave bearing surfaces centers balls 801,802 on axis 803.

Balls 801,802 are preferably stainless steel J2100, and housing 806 andmountings 220, 222 are preferably common steel. However, variousalternative materials could be used as in the case of the actuator ofthe preferred embodiment.

Rotor housing 806 is roughly a hollow cylinder, closed at one end,having a central shaft portion for engaging balls 801,802 and a flangefor supporting disk 807 from below. Rotor housing is preferably steel toprovide a magnetically permeable back iron for the permanent magnets.However, other materials such as plastic or aluminum may also be used,with or without a separate back iron member. A clamp mechanism 831applies an axial force downward on disk 807, pressing it against theflange and holding it in place. Where multiple disks are used, spacersare interposed between each disk and the clamping mechanism presses theentire disk stack against the flange. Various clamping mechanisms andspacers are known in the art.

Housing 806 is hollow for placement of motor components. A set ofpermanent magnets 832 is fastened to the inside of rotor housing 806. Anelectro-magnetic stator, comprising a core 834 and wire windings 835, ispositioned surrounding axis 803 within the space formed by housing 806.Motor core 834 preferably comprises a series of laminations of amagnetically permeable material, such as silicon steel. Motor coils orwindings 835 surround core 834 to form the stator electromagnet. Thestator is divided into a plurality of circumferentially spaced poles, asis known in the art. Permanent magnets 832 are arranged as a pluralityof poles of alternating polarity surrounding the stator, as is known inthe art.

While the electric motor of FIG. 8 employs electro-magnetic stator coilsdriven by alternating electrical current and permanent magnet rotors, asare conventionally used in disk drives and other small electric motors,it will be understood by those skilled in the art that an electric motorcould employ any of various means for imparting a torque to the rotor inresponse to an electro-magnetic field. For example, the rotor couldcomprise a set of closed loop coils as are commonly used in “induction”motors.

The bearing assembly of the present invention could also be used inapplications other than disk drives, where miniaturization, shockresistance, and cost reduction are significant goals. An example ofanother such possible application is the use of the bearing assembly tosupport rotating spools of magnetic tape in miniature magnetic tapecartridges.

Due to the reduced part count and simplicity of design, the bearingdesign of the present invention could be used in micro-mechanicalapplications, i.e., applications in which the sizes of moving parts aremeasured in microns. In present micro-mechanical applications, it iscommon to avoid the use of any type of ball or roller bearing, allowingrotating surfaces to directly contact stationary surfaces. As a result,the longevity of such parts is very limited. The use of a simple ballconfined by concave surfaces could dramatically increase the longevityand reduce drag in such applications. Typically, bearing surfaces insuch applications would be etched rather than machined.

In the preferred embodiment, a stamped steel pre-load spring is used toimpart a controlled axial pre-load force to the bearings. However, itwill be understood that a pre-load may be obtained through any ofvarious alternative means. A spring may be constructed of helical orconical wire. Alternatively, a compressible material such as foam rubbermay be used in place of the spring. The spring could also be mounted inother locations, e.g., on the rotor. As another alternative, the housingitself may be sufficiently elastic to provide a pre-load withinacceptable limits without the use of any spring or auxiliarycompressible material.

A bearing pre-load is usually considered desirable to give stiffness tothe bearings and enhance their accuracy. If the bearings are notpre-loaded, the non-repeatable runout of the rotating assembly would bevery large, i.e., the rotating assembly would tend to wobble in anunpredictable manner. This wobbling would probably make a bearingwithout pre-load unsuitable for application in an actuator or spindlemotor of a disk drive, due to the need to follow very narrow tracks ofdata to a high degree of accuracy. However, there may be applications inwhich bearing pre-load is not required or not desirable. Bearings whichare not pre-loaded would have reduced accuracy of alignment and reducedstiffness, but might also be cheaper, require even less space, and havereduced stiction. Therefore in an alternative embodiment of the presentinvention the bearings would not be pre-loaded.

In the description above, certain features have been referred to as“above” or “below” the actuator, or described as “upper” or “lower”.These terms are used only for ease of reference and are consistent withthe drawings and the normal orientation used in the art. However, theuse of these terms is not meant to imply that the present inventionrequires any particular orientation, e.g. the pre-load spring to belocated above the actuator. The disk drive of the present inventioncould just as easily be constructed with the pre-load spring locatedbelow the actuator, or with the axis of rotation oriented horizontally.Additionally, the words “pivot” and “rotate” have been usedinterchangeably to describe turning or spinning motion about an axis;unless limited by the context, these words should be understood toinclude motion in which the object turns a full 360 degrees as well asmotion in which the object turns only through an arc of less than 360degrees.

Although a specific embodiment of the invention has been disclosed alongwith certain alternatives, it will be recognized by those skilled in theart that additional variations in form and detail may be made within thescope of the following claims.

What is claimed is:
 1. A electric motor, comprising: a stationary base;an electromagnetic stator attached to said base; a bearing assembly forsupporting a rotor, said rotor rotating about an axis in successive 360degree turns, said bearing assembly comprising: (a) a first freelyrotating ball and a second freely rotating ball, said balls beingcentered on said axis and axially separated; (b) a first stationaryrotor mounting, said first mounting having a first concave bearingsurface centered about said axis and in contact with said first ball;(c) a second stationary rotor mounting, said second mounting having asecond concave bearing surface centered about said axis and in contactwith said second ball; and a rotor comprising: (a) a rotor housing; (b)means for imparting torque to said rotor in response to anelectro-magnetic field generated by said stator; (c) a third concavebearing surface centered about said axis and in contact with said firstball, said third concave bearing surface opposing said first concavebearing surface along said axis, said first and third concave bearingsurfaces confining said first ball, and (d) a fourth concave bearingsurface centered about said axis and in contact with said second ball,said fourth concave bearing surface opposing said second concave bearingsurface along said axis, said second and fourth concave bearing surfacesconfining said second ball; wherein said third concave bearing surfaceand said fourth concave bearing surface are positioned between saidfirst concave bearing surface and said second concave bearing surface,said rotor being supported entirely by said first and second balls. 2.The electric motor of claim 1, further comprising means for imparting acontrolled axial pre-load to said first and second balls.
 3. Theelectric motor of claim 2, wherein said means for imparting a controlledaxial pre-load comprises a compressible spring supporting said secondstationary mounting, said compressible spring imparting a controlledpre-load to said first and second balls.
 4. The electric motor of claim1, wherein said means for imparting torque to said rotor in response toan electromagnetic field comprises a set of permanent magnets attachedto said rotor housing.
 5. The electric motor claim 1, wherein said rotorsupports at least one recording disk for recording magnetically encodeddata.
 6. The electric motor claim 1, wherein at least one of said firstconcave bearing surface and said third concave bearing surface containsa curved portion thereof, said curved portion thereof being in contactwith said second ball at a location on said axis.
 7. The electric motorclaim 6, wherein at least one of said second concave bearing surface andsaid fourth concave bearing surface contains a curved portion thereof,said curved portion thereof being in contact with said second ball at alocation on said axis.
 8. The electric motor of claim 1, furthercomprising: a first hollow cylindrical shroud surrounding said axis andencircling said first freely rotating ball; and a second hollowcylindrical shroud surrounding said axis and encircling said secondfreely rotating ball.
 9. The electric motor of claim 8, wherein: saidfirst hollow cylindrical shroud extends from said first stationary rotormounting to encircle a first portion of said rotor; and said secondhollow cylindrical shroud extends from said second stationary rotormounting to encircle a second portion of said rotor.
 10. The electricmotor of claim 1, wherein: said first concave bearing surface and saidthird concave bearing surface contain respective curved portions thereofhaving respective radii of curvature near the intersection of said axiswith the respective curved portion, said curved portions being incontact with said first ball at respective locations on said axis, saidradii of curvature of said first concave bearing surface curved portionand said third concave bearing surface curved portion being greater thanthe radius of said first ball; and said second concave bearing surfaceand said fourth concave bearing surface contain respective curvedportions thereof having respective radii of curvature near theintersection of said axis with the respective curved portion, saidcurved portions being in contact with said second ball at respectivelocations on said axis, said radii of curvature of said second concavebearing surface curved portion and said fourth concave bearing surfacecurved portion being greater than the radius of said second ball. 11.The bearing assembly of claim 10, further comprising means for impartinga controlled axial pre-load to said first and second balls.